1. Field of the Invention
The present invention relates to the measurement of material web characteristics, and, more particularly, to sensors used in the measurement of material web characteristics.
2. Description of the Related Art
In measuring a basis weight, moisture or thickness, or other characteristics, of a moving material web a radiation source is mounted on one side of the web material and on the other side is a receiving transducer. The source/transducer pair are then scanned in a cross direction (perpendicular to the movement or machine direction of the web material) to provide a cross direction measurement of that particular characteristic.
Any uncompensated variation in the size of the measuring gap will cause an error in the measurement. In the past, these errors were compensated either by using a separate sensor to measure the gap size, with additional references and the like or by shaping the radiation beam so that the sensitivity to alignment errors were minimized. The disadvantage of using separate sensors or additional references to measure the gap size is the increase in cost and complexity, and a corresponding decrease in reliability, associated with the additional equipment. The disadvantage in shaping the beam is the associated reduction in signal strength leads to a reduction in the signal to noise ratio.
U.S. Pat. No. 5,233,195 discloses methods and apparatus for measuring characteristics of moving webs wherein alignment insensitivity is obtained by tuning the radiation beam using concentric rings or crossed strips of material which is semi-transparent to the radiation to compensate for geometric characteristics of the radiation source/beam and the detector. U.S. Pat. No. 5,010,766 discloses error compensation for measuring gauges by the use of two pairs of eddy current sensors which are centered on a target on the opposed transducer head and which sense the pairs of edges of an aperture in the opposed transducer. U.S. Pat. No. 4,678,915 discloses a system for measuring the values of a parameter of a web of material including a head system with sensors mounted therein and a separate distance correction system. The separate distance correction system corrects the measured parameter for variations in the distance between parts of the head system using a coil circuit. U.S. Pat. No. 4,276,480 discloses a sensor response indicative of a distance which can be related to a thickness property of a material by a procedure that includes establishing a reference position, such as to a support, and locating the material precisely in a predetermined spatial relation to the reference position.
What is needed in the art is a sensor which is insensitive to both lateral and vertical misalignment between the radiation source and receiver and which does not require additional detecting elements, references or beam shaping.